1. Field of the Invention
This invention relates to improvements in a control system for DC motors of the type employing permanent magnets as field poles.
2. Description of the Prior Art
In a DC motor of the type employing permanent magnets as field poles, an abnormal increase in an armature current is likely to cause demagnetization of the permanent magnets by the armature reaction.
A method that has been adopted in the prior art for protecting the permanent magnets from demagnetization is to check the armature current and clamp an error voltage to be applied to a thyristor control circuit in the case of the armature current exceeding a predetermined value, thereby preventing a further increase in the firing phase angle of a thyristor to avoid a further increase in the armature current.
With this method, however, since an abnormal current detecting level of a current detector is set constant independently of actual motor speeds, demagnetization of the permanent magnets cannot be stably prevented over the whole range of the motor speed; therefore it is impossible to completely remedy the drawback of a delay in the operation of the current detector which causes the armature current exceed its rated value, resulting in the permanent magnets being demagnetized.